Recently, as the portability and cordless tendency of instruments have progressed, a demand for a non-aqueous electrolyte secondary battery such as a lithium secondary battery which is small in size and light in weight and has a high energy density, has been increasingly high. As a cathode active material for the non-aqueous electrolyte secondary battery, a composite oxide of lithium and a transition metal or the like (which may be referred to as a “lithium-containing composite oxide” in the present specification) such as LiCoO2, LiNi1/3Co1/3Mn1/3O2, LiNiO2, LiNi0.8Co0.2O2, LiMn2O4 or LiMnO2, has been known.
A lithium secondary battery using a lithium/cobalt composite oxide (LiCoO2) as a cathode active material and using a lithium alloy or carbon such as graphite or carbon fiber as a negative electrode, can obtain a high voltage at a level of 4 V, whereby it has been widely used as a battery having a high energy density. However, a cobalt source which is a material of the lithium/cobalt composite oxide is rare, and it is expensive. Further, in the case of the lithium secondary battery using the lithium/cobalt composite oxide as a positive electrode, it is very unstable under the charging state, it is ignitable, and its low stability is a substantial problem.
Accordingly, as the cathode active material, attention has been paid to a lithium/nickel/cobalt/manganese (Li—Ni—Co—Mn)-containing composite oxide which is not expensive, of which materials are naturally abundant elements such as manganese and which is stable under the charging state and highly safety. However, one satisfying various properties such as the discharge capacity per unit weight (which may be referred to as discharge capacity or weight capacity density in the present specification), packing property, cycle property, discharge rate property, safety and coating property relating to a coating state on a current collector, etc. at a time of producing batteries, has not been obtained.
In order to solve these problems, various studies have been made heretofore as described below.
For example, in order to improve the discharge capacity and the discharge rate property, it has been proposed to use lithium/nickel/cobalt/aluminum-containing composite oxide particles obtained by mixing a powder having a large average particle size and a powder having a small average particle size, of which compositions are the same (Patent Document 1).
Further, it has been reported that when the proportion of lithium is controlled within a range of the molar ratio of from 1.06 to 1.3 based on the total amount of nickel, cobalt and manganese contained in the lithium/nickel/cobalt/manganese-containing composite oxide, the discharge capacity and the cycle property are improved (Non-Patent Document 1 and 2).    Patent Document 1: JP-A-2003-051311 (Claim 1, pages 12 and 13)    Non-Patent Document 1: Electrochimica Acta, June 2006, vol. 52, p. 1483-1490    Non-Patent Document 2: Journal of Power Sources, September 2006, vol. 162, p. 629-635